The present invention relates to atrium-controlled pacemakers. In such pacemakers, stimulation of the heart occurs with a variable pulse repetition rate which depends upon the appearance of atrial signals. The pulse repetition rate may vary, for example, between 50 and 150 pulses per minute.
It is an object of the present invention to provide an atrium-controlled heart pacemaker such that physiologically unfavorable time intervals between atrial and ventricular activities are avoided in a Wenckebach mode wherein the spacing between two ventricular stimulation pulses or between a natural activity (QRS sequence) and a stimulation pulse is continuously varied.
The above object is achieved in accordance with the principles of the present invention by selecting a smallest synchronous interval (SSI) between two ventricular stimulation pulses or between a natural activity sequence and a stimulation pulse, as well as selecting a minimum and maximum atrial-ventricular delay time AVmin and AV max. A control unit is provided which, dependent upon the time of appearance of an atrial signal, controls operation of the heart pacemaker in one of several modes. If the atrial signal appears after a point in time SSI-AVmin, the heart pacemaker is operated with a variable pulse rate (DDD mode). When the atrial signal occurs between the point in time SSI-AVmin and the point in time SSI-AVmax, the control unit operates the heart pacemaker with a highest synchronous pulse rate. No stimulation pulses are produced when the atrial signal appears before the point in time SSI-AVmax.
In the pacemaker disclosed herein, a Wenckebach function is provided in that the AV delay is varied within a narrow, defined region, whereas the AV delay is lengthened only in the Wenckebach mode. The additional delay can become so large that the AV synchronism is lost during the Wenckebach mode. The Wenckebach function of the pacemaker is universal. If AVmin is established equal to the normal AV delay, the Wenckebach function of the pacemaker changes into a normal Wenckebach function in which a maximum delay is present.